The present invention relates generally to AC power line test equipment and, more particularly, to a power line testing apparatus capable of transmitting a tracer signal easily distinguishable from environmental noise.
When electrical power line outlets (receptacles) are installed in residential, commercial or industrial environments, it is extremely important that they are properly wired. If such outlets are incorrectly wired, they may be absolutely useless and/or can cause significant damage to equipment and property or, even worse, they can result in electrical shock, thereby possibly causing serious harm to, or death of, individuals. A further extremely hazardous situation occurs, even if the wiring scenario is correct, but where the integrity of the connected earth ground conductor is compromised. A proper ground connection is meant to shunt leakage current, which can appear at conductive enclosures of appliances due to defects or poor insulation, to earth ground. The integrity of earth ground conductors can be compromised by an increase in electrical impedance due to corrosion of conductors and/or conduits (when used as earth ground) or by degradation of conductor-to-conductor connections in commonly used wire nuts or screw terminals, which may loosen over time. If sufficient impedance is present in a ground system, the ground conductor is no longer a real earth ground.
Ground fault interrupting devices are known in the art. Those devices are usually equipped with an internal test function which works quite reliably, yet they cannot test the integrity of fixed or temporary branch circuit extensions. Therefore, numerous external testing devices have become available which are to be connected to such extensions and to perform the required test from there. Almost all known devices in this art generate a leakage current flow of a certain amount from the hot to the ground conductor, but they ignore the time that elapses until the device under test reacts. This leads in many cases to the false assumption that the device under test works properly and reliably while, in fact, in many cases it does not since the device indeed reacts, but exceeds the permissible period of time for reaction.
Fairly new in the art are so-called arc-fault interrupting devices. These devices are meant to interrupt an electrical branch circuit if serial or parallel arcing occurs along or between hot and neutral conductors in the power line system. Like ground fault circuit interrupting devices, they usually have an internal test function. In practice, there is a significant problem with these test functions because the activation of such a test is limited to either giving an incorporated microcontroller a command to activate the associated trip mechanism, or xe2x80x9cinjectxe2x80x9d an electronic signal to the system in order to simulate arcing. As a result of the first such xe2x80x9ctestxe2x80x9d, it only gets proven that the microcontroller is xe2x80x9calivexe2x80x9d and that the mechanical parts of the apparatus work properly. A second type of xe2x80x9ctestxe2x80x9d is apparently more enhanced than the first. However, in both of these tests, there is no reliable confirmation whatever that the devices will work properly and reliably under real arcing conditions.
When work needs to be performed on an electrical branch circuit, it is first necessary to unenergize that circuit or interrupt the current flow to the circuit. This is commonly achieved by opening or unscrewing the circuit-interrupting device in the distribution panel that is associated with that particular branch circuit. In many cases, it is not known which circuit-interrupting device, out of a plurality of such devices (as commonly occurs on a panel of circuit breakers), is actually the one in question. Absolute methods of determining the correct device are often not only inconvenient and time consuming but, in many circumstances, not even feasible. A xe2x80x9cclassicxe2x80x9d method used to find the associated device needed is to sequentially unscrew fuses or to open circuit breakers in a distribution panel until the one in question has been found. Subsequently, all outlets in the branch circuit under test need to be checked for an unenergized condition. In certain environments, such as hospitals or manufacturing plants, this or similar methods are totally impractical. In other environments (e.g. where computers without backup power are used), such methods can be, at the very least, disturbing and/or annoying. As alternatives to these methods a variety of electronic devices have been developed that accurately determine which is the particular circuit-interrupting device in question. By examining those devices, it becomes apparent that they all have significant drawbacks.
The invention disclosed in Virgilio U.S. Pat. No. 5,625,285 describes a device for monitoring the present wiring scenario and acceptable grounding properties on a standard 3-Wire 120 volt AC electrical outlet. The circuit for this device is reproduced as FIG. 2. The described device can detect and indicate the following wiring possibilities:
1. Correct wiring
2. Defective ground
3. Open neutral
4. Hot and neutral reversed
5. Hot on neutral with open neutral
One of the most hazardous situations, hot and ground reversed, cannot be detected and indicated. Further, it sometimes occurs that a second hot wire is mistakenly connected to a receptacle. In such a case, the voltage across the two hot conductors is twice that of the nominal voltage. If someone intends to perform work on such a miswired circuit, he or she faces an undesirable and potentially dangerous situation, since the circuit is still energized even if one associated circuit-interrupting device has been deactivated.
In order for the Virgilio structure to analyze the integrity of the earth ground conductor, a high current pulse of short duration is drawn over the power line system. A pickup coil senses the strength of the thereby generated magnetic field. The induced voltage in the pickup coil is then used to trigger a semiconductor device, which then activates a visible indicator. This circuit requires exact calibration during the manufacturing process. If parasitic impedance is present in the ground conductor, the magnetic field loses strength, therefore the induced voltage in the pickup coil is no longer sufficient to trigger the semiconductor control device. In practice, line resistance and capacitance are subject to continuous changes due to frequent on and off switching of heavy electrical loads. This has major impact on the proper performance of that device. These line and ground impedances impact the reliability of Virgilio""s ground integrity test.
Another structure, disclosed in Robitaille U.S. Pat. No. 4,929,887, describes an electrical outlet monitor that recognizes the fact that more than one hot conductor can be, possibly, connected to an outlet. However, the unit does not indicate in detail what the present wiring scenario is, but only that it is incorrect. The circuit for this unit is shown in FIG. 1.
No patented prior art is known to the applicant which covers GFCI testing devices that include the measurement and consideration of elapsed time as part of testing criteria. However, there is a device available, designed and manufactured by a German company named BEHA GmbH and distributed in North America by Greenlee which does take elapsed time into consideration.
Another structure, disclosed in Spencer et al. U.S. Pat. No. 5,875,087, describes an enhanced digital circuit breaker that includes an AFCI and an associated AFCI test function. As it is apparent from the block diagram of this breaker (FIG. 14), activation of the incorporated momentary push button 20 does not in any way introduce an actual arcing condition in the power line system under test, but rather, simply instructs the microcontroller to mechanically trip the device through D2 and thyristor 22. In effect, this is not at all a xe2x80x9ctestxe2x80x9d of the reliability of the device to respond to a genuinely dangerous condition; it is simply a demonstration of the xe2x80x9cdesiredxe2x80x9d response to such a dangerous condition.
A structure disclosed in MacKenzie et al. U.S. Pat. No. 5,459,630 depicts an enhanced circuit breaker with incorporated GFCI and AFCI functions. It also includes a xe2x80x9cself-testxe2x80x9d for those two functions. This self-test function still fails to introduce an actual arcing condition to the power line system for the purpose of testing. MacKenzie et al. depicts two different ways by which the incorporated arc-fault detector can be tested. First, it generates an electronic signal with pre-determined parameters in order to electronically simulate an arcing condition, thereby forcing the arc-fault detector to respond in the desired way. Secondly, and in a nearly similar way, it generates a test signal of a different shape and takes more parameters into consideration. Here, again, there is no real or actual arcing introduced into the power line system for the purpose of testing the reliability of the AFCI.
U.S. Pat. Nos. 4,906,938 (Konopka), 5,497,094 (George), and 5,969,516 (Wottrich) all introduce devices consisting of two separate units in order to locate a particular circuit-interrupting device, among a number of such devices in an electrical distribution panel. All three devices draw a current spike over the power line system of sufficient strength and short duration that it can be used as an identification signal. The associated receivers for all three devices require manual adjustment in order to evaluate the detected identification signal in quantitative terms. None of those devices has the capability to evaluate an identification signal in terms of xe2x80x9cqualityxe2x80x9d. Power line systems are, to a great degree, polluted by numerous and various electrical signals that are generated arbitrarily, accidentally, or deliberately.
Many of those signals are sharp rising and of short duration, thereby having very similar shapes as the identification signals of the above-mentioned three devices. Since, as mentioned, those three devices do not evaluate any detected signal in terms of quality, they often get triggered by any number of polluting signals rather than by the intended identification signal. This makes those units, to a certain degree, unreliable. The technologies disclosed in U.S. Pat. Nos. 4,906,938, and 5,497,094 are of quite simple design, while the technology disclosed in Wottrich U.S. Pat. No. 5,969,516 is somewhat more enhanced. The transmitter of the Wottrich device has a wiring monitor incorporated that is, to some degree, similar to the wiring monitor disclosed in Virgilio U.S. Pat. No. 5,625,285 (as mentioned above). Furthermore, this structure is claimed to have the capability to generate an identification signal under all scenarios that are detected and indicated by the incorporated wiring monitor, except an unenergized circuit. In fact, it generates, under some wiring conditions, not only one but two identification signals. This gives rise to false identification. Practical evaluations of this device show that GFCI units, if associated with a branch circuit under test, get tripped immediately when the transmitter of this device gets connected to the electrical branch circuit.
FIGS. 11 and 12 show the schematics of two conventional devices for locating circuit-interrupting devices associated with a particular electrical branch circuit. The design in FIG. 11 is of a quite simple structure and is disclosed in U.S. Pat. No. 4,906,938. U.S. Pat. No. 5,969,516 discloses a design that is illustrated in FIG. 12. As mentioned above (prior art), this device can generate an identification signal under different wiring scenarios, yet has the drawback as depicted above.
The intended purpose of the herein described invention is to provide a tool, which can be used by either a professional or a layman, that allows work to be done on a power line system in the safest manner possible. This invention is an electronic testing device that includes two separate units. These units can be used independently or in conjunction with each other.
This device provides the following specific functions:
It is an AC outlet wiring monitor that analyzes and indicates the wiring scenario present in a standard power line system, testing for both correct wiring and the most common of the various incorrect wirings.
It is a self-adjusting testing unit with regards to different power line systems (110V/60 Hz, 220V/50 Hz, 440V/50 Hz), or any other situation caused by improper wiring.
It analyzes and indicates the integrity of a given earth ground system by measuring the amount of parasitic ohmic resistance.
It provides the capability to test the reliability of an associated GFCI by not only generating a leakage current across the hot and ground conductor, but also measuring the time that elapses until reaction as an essential part of the testing criteria.
It provides the capability to test the reliability of an associated AFCI by generating a real, physical, sputtering arc in the power line system while also measuring the time that elapses until reaction as an essential part of the testing criteria.
It is a device that can be used to locate a particular circuit-interrupting device (E.g. circuit breaker or fuse) associated with the particular electrical branch circuit under test. Unlike all other known units, this device does not require any manual adjustments. Also, unlike all other known units, this device analyzes the quality of the generated identification signal, in addition to the quantity, in order to avoid false indications.
It senses the presence of an electrical field in common 50/60 Hz power systems and indicates the strength of the field by increasing and/or decreasing the duty cycle of a visual and/or audible indicator.
According to one aspect of the present invention, a signal generator adapted to be placed to on a branch circuit includes a spark gap and a switch connected in series with a spark gap. When the switch is actuated, an arc will appear across the spark gap, thereby injecting an actual arcing condition into the circuit to be tested. Preferably, the switch is an optoisolator that is controlled by a logic circuit, which, in turn, is manually operated by a user.
According to another aspect of the invention, the power circuit testing device transmitter includes a transformerless power supply which can derive DC power from any AC electrical outlet, even one which is miswired, as long as one of the conductors is energized and another of the conductors provides a return path. This power supply uses full wave rectification.
According to a third aspect of the invention, a signal transmitter of a branch circuit locating system generates a signal in which the period of the signal is different from a period of an AC carrier on which the signal resides. This permits the easy identification of the signal as coming from an artificial source and makes it easy to discriminate this signal from other system conditions. In a preferred embodiment, the signal is amplitude-modulated and consists of voltage spikes on maxima and minima of the carrier wave through two cycles, with an intentional omission of such spikes on at least one following cycle. The signal generated according to the invention preferably has both negative and positive going components.
According to yet a further aspect of the invention, the power line testing device according to the invention determines the integrity of the ground connection. It does this by measuring the parasitic impedance of the ground connection. In a first embodiment of the ground integrity test function, this is done by a voltage divider. In a second embodiment of the ground integrity test function according to the invention, a phase shift is measured taking into account both real and imaginary components of the parasitic ground impedance, and the degree of phase shift of a varying signal is ascertained and compared against a stored criterion.
According to yet another aspect of the invention, the transmitter according to the invention is capable of producing a fault signal across any two of the three conductors typically found in an AC power circuit, as long as the first of those conductors is energized and a second of those conductors provides a return path.
According to yet a further aspect of the invention, a receiver of the testing system measures incoming signals over a predetermined period of time and averages signal strength. This signal strength average is compared against a stored reference in order to determine whether or not the AFCI or GFCI interrupters are correctly operating.